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A 72 year old man who had never undergone joint surgery was referred to an orthopaedic clinic because of pain and stiffness in his right hip, which had been gradually worsening for many years. He was known to have hereditary multiple exostoses but was otherwise well. He had not noticed any recent new bony lumps. Pelvic radiography was performed (fig 1⇓).

Osteochondromas are the most common bone tumour, comprising up to half of benign bone growths. They are composed of cortical and medullary bone in continuity with that of the underlying bone, with a hyaline cartilage cap. The long bones of the lower limb are most commonly affected, although lesions can be seen in the upper limbs, scapular bone, pelvis, spine, and head.12 The nature of osteochondromas is unclear, and many clinicians treat them as if they were tumours. However, recent studies suggest that they consist of a non-clonal mixture of cells and are not true cancers.3 The lesions arise by separation of a fragment of the epiphyseal growth plate that herniates through the periosteal bone cuff surrounding the growth plate …